1. Field of the Invention
The present invention relates to a display device, and more particularly, to a gamma reference voltage generating circuit and a method of using a gamma reference voltage generating circuit in a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for obtaining an optimized luminance in a transmissive mode and a reflective mode.
2. Discussion of the Related Art
A gamma reference voltage generating circuit of a liquid crystal display is an essential element of the liquid crystal display that influences picture quality. The gamma reference voltage generating circuit generates and outputs a reference voltage required for digital/analog conversion in a source driving circuit.
FIG. 1 illustrates the structure of a liquid crystal display device according to the related art. In FIG. 1, the liquid crystal display device includes a liquid crystal display 11, a gate driving circuit 12, a source driving circuit 13, and a gamma reference voltage generator 14. The liquid crystal display panel 11 includes a plurality of gate lines arranged at fixed intervals along a first direction, and a plurality of data lines arranged at fixed intervals along a second direction orthogonal to the gate lines, thereby forming a pixel region in a matrix array. The gate driving circuit 12 outputs a pulse signal, which sequentially scans pixels of the liquid crystal display panel 11 column by column. The source driving circuit 13 converts externally input red (R), green (G), and blue (B) digital video signals into analog signals, and outputs the converted video signals to each of the plurality of data lines. In order to convert the R, G, and B digital video signals into analog signals, a digital/analog conversion is performed using a reference voltage output from the gamma reference voltage generator 14, thereby generating a liquid crystal driving voltage. The generated liquid crystal driving voltage is applied to the plurality of data lines of the liquid crystal display panel during each scan.
The gamma reference voltage generator 14 serially connects a plurality of resistors between a power terminal Vdd and a ground terminal, thereby supplying a divided voltage. Furthermore, the gamma reference voltage generator 14 generates and outputs the reference voltage necessary for converting the digital video signals at the source driving circuit 13.
FIG. 2 shows a block diagram of a source driving circuit according to the related art. In FIG. 2, the source driving circuit includes a shift register 1 outputting a latch clock signal, a first latch unit 2 respectively latching R, G, and B digital video data signals, which are sequentially synchronized with clock signals of a timing controller (not shown), and converting a timing system signal of a dot-at-a-time scanning into a line-at-a-time scanning in accordance with the latch clock signal output from the shift register 1, a second latch unit 3 latching data stored in the first latch unit 2 at every horizontal line cycle in accordance with a transfer enable signal, a digital/analog converter 4 converting the data latched by the second latching unit 3 into analog signals in accordance with the gamma reference voltage, and a buffer 5 buffering the analog signals output from the digital/analog converter 4 and outputting the signals to each data line.
Since the picture quality of the liquid crystal display is highly dependent upon the gamma reference voltage, the gamma reference voltage should be determined based on the electro-optical characteristics of the liquid crystal display panel. A liquid crystal display may be classified, based upon the backlight device used, into a transmissive mode, a semi-transmissive mode, and a reflective mode. The semi-transmissive mode of the liquid crystal display may perform either of two different driving modes depending on the operating conditions. More specifically, a first driving mode includes the reflective mode using a peripheral light source, and a second driving mode includes the transmissive mode using a backlight source. However, due to differences in transmission and reflection characteristic curves of the two driving modes, luminance of the liquid crystal display may vary depending on external conditions, thereby deteriorating picture quality.
FIG. 3 shows a luminance curve of the transmissive mode and the reflective mode according to the related art. In FIG. 3, the luminance value of the transmissive and the reflective modes may be explained by using the following equations:L*=116(Y/YMAX)⅓−16forY/YMAX>0.008856L*=903.3(Y/YMAX)forY/YMAX≦0.008856where L* represents the luminance value considered the human visual characteristic, Y represents the luminance value at gray scales, and YMAX represents the maximum luminance value.
The gamma reference voltage is determined by generating a gray voltage in accordance with the maximum luminance value YMAX. More specifically, as shown in FIG. 3, when using the source driving circuit that displays 64 gray scales, the difference in LT values between each gray scale is about 1.25 ((100-20)/64) in the transmissive mode and about 1.0937 ((100-30)/64) in the reflective mode. Therefore, a middle gray scales (i.e., 32 gray scales) can be described by using the following equations:LT(X)=1.25×X+20LR(X)=1.0937×X+30where X is the number of gray scales.
The LT value is about 60 in the transmissive mode, and the LR value is about 64.9 in the reflective mode. In such cases, as shown in FIG. 3, the driving voltage is 2.2V in the transmissive mode and 2.35V in the reflective mode. Accordingly, a difference in driving voltage occurs between the transmissive mode and the reflective mode in an identical gray scale, which is the middle gray in this case. Therefore, when the transmissive mode and the reflective mode are operated with the same gamma voltage circuit, differences occur in the gray scale that is actually realized. Accordingly, the gamma reference voltage circuit of a liquid crystal display according to the related art has the following disadvantages. When determining a gamma reference voltage according to a difference in luminance in the reflective mode and the transmissive mode, either a compensated value of the two curves or a design value of a compensating film used in designing the panel was modified. However, such solutions are insufficient for determining the gamma reference voltage value in the liquid crystal display panel.